The invention relates to highly-crosslinked polysiloxane gels made from non-shrinking siloxane materials and a process for making the same. More particularly, this invention relates to a new class of disilaoxacyclopentane monomers that do not shrink substantially when undergoing a ring-opening polymerization to form highly-crosslinked polysiloxane gels useful as encapsulants.
In the general sol-gel chemistry of alkoxysilanes to form highly-cross-linked polysiloxane gels, stoichiometric quantities of water are used as part of a step-growth polymerization process and solvents are used for monomer/water miscibility. Subsequent evaporation of the solvent results in fragile gels as well as shrinkage of up to 90%. The drying process itself must be carried out slowly to avoid cracking of the gel. Much of the shrinkage results from the evaporation of the solvent used in the process and from by-products of the polymerization reaction itself. For example, Gloffelter et al. (U.S. Pat. No. 5,120,811, issued on Jun. 9, 1992) describe a sol-gel process for producing a polymer/glass hybrid coating that encounters significant changes in volume of up to 8:1 during the drying stage. Likewise, Haruvy et al. (U.S. Pat. No. 5,272,240, issued on Dec. 21, 1993), in another method for the sol-gel preparation of glasses, note that sol-gel polymerization reactions commonly result in cracking and fragmentation due to the extensive volume-contraction which accompanies the condensation reaction and the corresponding expulsion of the solvent and the condensation products. Haruvy et al. address this problem in part by choice of monomer and choice of reaction conditions as the hydrolysis reaction proceeds. By-products are still produced and water and optional solvent are still added during the hydrolysis reaction step. Curing at room temperature takes from hours to days.
One method for reducing shrinkage is to eliminate solvent and condensation by-products by replacing the step growth polymerization with a chain growth polymerization such as ring opening polymerization (ROP). Sadhir et al. (Sadhir, R. K. and Luck, R. M., "Expanding Monomers: Synthesis, Characterization and Applications," 1992, CRC Press, pp. 21-37) has shown that ROP is an effective means for reducing or, as with the polymerization of spiroorthocarbonates, completely eliminating, shrinkage in linear, hydrocarbon polymers. Suryanarayanan et al. (Suryanarayan, B., Peace, B. and Mayhan, K., J. Polym Sci.; Chem. Ed., 1974, 12, 1089) and Samara et al. (Samara, M. and Loy, D., Polym. Preprints, 1998, 39(1), 599) have shown that the simple monomer 2,2,5,5-tetramethyl-2,5-disilaoxacyclopentane can form a polymer under ROP in the presence of tetrabutylammonium hydroxide with shrinkage of less than 5%.
Useful would be highly-crosslinked gels that exhibit essentially no shrinkage and whose synthesis results in no condensation by-products. Loy et al. (Loy, D. A., Rahimian, K. and Samara, M., Angew. Chem., Int. Ed., 1999, 38, 555; incorporated herein by reference), Rahimian and Loy (Rahimian, K. and Loy, D., Polymer Preprints, 216th National Meeting of the American Chemical Society, Boston, Mass. 1998; incorporated herein by reference), and Rahimian and Loy (Rahimian, K. and Loy, D., Polymer Preprints, 217th National Meeting of the American Chemical Society, Anaheim, Calif. 1999; incorporated herein by reference) discuss novel monomers useful in preparing highly-crosslinked gels that exhibit little or no shrinkage during a ring opening polymerization process. The crosslinked gels produced have no porosity or surface area and can be used as encapsulants as well as coatings. The process used involves no solvents or water addition.